if I would use the Mezmerize B1 Buffer Preamp in my DAC instead of dual opamps, what changes do I have to do, no vol.pot etc. DAC-chip is 4x PCM1704, 4x op-amp INA103. Of course I need 2 boards for balanced signal.

That one you would be better off asking in the digital forums that they are familiar. So they can probably reply if 4 JFET DC coupled buffer channels are what you can substitute in your application. That is what two Mez boards will provide along with input selection bays that can be left unused, signal can be fed with no vol pot also.